This project seeks to determine if prenatal air pollution exposures increase the risk of cognitive delays and autistic traits. Emerging evidence suggests that air pollutant exposure may increase risk of neurodevelopmental disorders and autism spectrum disorder (ASD). Our research demonstrates that exposure during gestation and early life to traffic related air pollution (TRP) and regional particulate matter (PM) were associated with risk of autism and neurodevelopmental delays. Other studies indicate neurodevelopmental effects based on prenatal biomarker measures of polycyclic aromatic hydrocarbon (PAH) exposure. However, we do not know if these associations persist over time, nor has a critical period of development been pinpointed. In this study we propose to examine the relationship between prenatal air pollution exposure and early longitudinal measures of cognitive ability and ASD in two prospective samples - Markers of Autism Risk in Babies, Learning Early Signs (MARBLES) and Early Autism Risk Longitudinal Investigation (EARLI). MARBLES and EARLI are both longitudinal studies of pregnant mothers, who have had one child with ASD, increasing the risk for abnormal development and ASD in the subsequent child. Approximately 40% of the infant children from MARBLES and EARLI will have a DD and 1/7 will be diagnosed with an ASD, which enriches our ability to study a broad range of neurodevelopmental outcomes. We will leverage the valuable phenotype and biospecimen resources of MARBLES and EARLI for the first prospective study of prenatal air pollution effects on autistic traits and the trajectory of cognitive development over the first thirty-six months of life. Specific aims are: (1) to assign air pollutant exposure using state of the art modeling techniques for multi- site studies estimating TRP and PM as well as measure novel biomarker measurements of nitro-PAH exposure to freeway-based diesel exhaust and gaseous nitro-PAH pollutants from vehicular and other sources; (2) examine the effect of these exposures on the trajectory of cognitive development using repeated administrations of the Mullen Scales of Early Learning (MSEL); and (3) evaluate the effect of these exposures on autistic traits and ASD diagnoses. Because air pollutant exposure is common and can be mitigated, the potential public health impact of this study is large. Additionally, identification of a risk factor in very young children provides the opportunity for early intervention where reduction of risk for disordered development is still possible.